The invention relates to a formed ellipsoidal adsorbent material, for example in the form of cylindrical or elliptical beads or rods, or the like, which can be used to purify or separate gas mixtures, especially gas mixtures that can be used in a PSA (Pressure Swing Adsorption) or VSA (Vacuum Swing Adsorption) process.
xe2x80x9cHigh productivityxe2x80x9d production cycles are characterized especially by a rapid alternation of the adsorption/desorption cycles, which amounts to adsorbing the amount of gas corresponding to the capacity of the adsorbent in a short space of time.
To do this, it is desirable to have adsorbents in which mass transfer between the gas phase and the adsorption sites is rapid.
To do this, it has already been proposed to reduce the size of the granular adsorbent particles or to improve the transport capabilities of the adsorbent particles with no, or little, change in their size.
It is also possible for the adsorbents to be in a monolithic form, for example in xe2x80x9choneycombxe2x80x9d form or xe2x80x9cmillefeuillexe2x80x9d form.
However, although in theory it is always possible to increase the rate of adsorption of an adsorbent by reducing the scale of contact between the gas phase and the solid phase or the surface-to-volume ratio of the adsorbent, this leads in general to adsorbent configurations creating many head losses. This therefore results in a considerable expenditure of energy and, consequently, is to the detriment of the industrial profitability of the production process using these adsorbents.
Adsorbents having a high adsorption rate are described, for example, in the patents WO-A-99/43415, WO-A-99/43416, WO-A-99/43417, WO-A-99/43418 and EP-A-0940174 which relate to processes using zeolite agglomerates whose kinetic properties have been improved for better use in VSA/PSA applications. However, the approaches described relate only to improving the transport properties of the particles.
A theoretical approach has been given by Perera et al. in Trans. IchemE, Vol. 76, Part A, November 1998, pp. 921-941 and by Ruthven et al. in Separation and Purification Technology, No. 12 (1997), pp. 43-60 and No. 13 (1998), pp. 181-193.
A comparison between the performance of monoliths and granular beds is presented therein. However, although the individual improvement factors are given, namely the channel diameter and bead diameter, the diffusivity of the adsorbent phase, thickness of the adsorbent layer, density of the adsorbent phase and adsorption properties, these documents provide no teaching as regards the optimization of these parameters and a person skilled in the art would, in practice not know which route to follow.
In fact, it appears that none of the routes explored is ideal as various problems are encountered when implementing them, such as excessive head loss problems in the case of small particles or practical processing problems in the case of monoliths, or else changes to the associated properties in the case of particles with improved porosity, such as, for example, the reduction in mechanical strength or the reduction in density when the porosity becomes too high.
Starting from this situation, the object of the invention is to propose an improved adsorbent that can be used in a PSA or similar process, which does not have the drawbacks of the prior art, and to do so by judiciously taking into account both the properties of the adsorbent material and its geometry with respect to the gas phase which ensures transport of the adsorbates.
In other words, the object of the invention is to propose an adsorbent whose geometrical and intrinsic characteristics result in its optimum use in rapid adsorption cycles for the production or separation of gas, and to do so whatever the particular form chosen for this adsorbent.
Put another way, the invention allows the transport properties of the material and its geometry to be precisely adjusted, taking into account other criteria such as head losses.
The invention therefore relates to a formed adsorbent material comprising an adsorbent solid phase and a gas phase ensuring transport of the gaseous components right to the adsorbent phase, the material being such that the constants for the transport kinetics of the components adsorbable in the gas phase and in the solid phase are in a ratio of between 0.1 and 10 defined for a gas discharge velocity of 0.2 m/s measured on air at 1 bar and 20xc2x0 C.
The invention also relates to a formed adsorbent material comprising an adsorbent solid phase and a gas phase ensuring transport of the gaseous components right to the adsorbent phase, the material being such that the constants for the transport kinetics of the components adsorbable in the gas phase and in the solid phase are in a ratio of between 0.1 and 10 defined for a gas discharge velocity of 0.01 m/s measured on hydrogen at 30 bar and 40xc2x0 C.
The invention also relates to a process for separating or purifying a gas mixture by adsorption of at least one gaseous compound from the gas mixture on an adsorbent material according to the invention, preferably a PSA or VSA process.
Depending on the case, the process may include one or more of the following features:
the gas mixture contains oxygen and nitrogen;
at least some of the nitrogen is adsorbed on the adsorbent material so as to produce an oxygen-enriched gas stream, the gas mixture preferably being air;
the gas mixture to be purified contains hydrogen and at least one impurity chosen from CO, CH4, CO2, O2, and N2;
at least some of the impurities are adsorbed on the adsorbent material so as to produce a hydrogen-enriched gas stream;
the gas mixture to be purified is air containing at least one impurity chosen from CO2 and H2O;
at least some of the impurities are adsorbed on the adsorbent material so as to produce purified air stripped of at least some of the impurities, the purified air obtained preferably being subsequently subjected to a cryogenic distillation step; and
the gas mixture contains at least hydrogen and CO.